Blackstone’s Byron Wien has been pretty bullish on stocks over the past couple years. But now he’s warning investors to be cautious.

In commentary available on Barrons.com , Wien says “I know there is a feeling among policymakers at the Fed that the current high degree of monetary accommodation is excessive and should be restrained when the economy has regained its natural momentum. I do not believe we have come anywhere near that point yet. Moreover, the Fed’s own targets for implementing a program of restraint have not been met. … Since the economy is running well below its potential, I believe the Fed should maintain its current level of security purchases. Any reduction in the program is a form of tightening and neither the market nor the economy is really ready for that.”

Wien sees profit margin peaking and a lot of economic concerns around the globe. He thinks the US economy is suffering from a lack of demand and structural problems in some areas, including employment.

Wien does see some reasons for optimism. One is a decline in the US budget deficit, which is down from 10 percent of GDP in 2010 to 4 percent this year, and likely to the decline next year, he says. He thinks that could give Congress room to put into action structural improvements that have been delayed for a long time.

“I still think the economy will grow at 2% this year, but I worry that final sales will not be enough to offset some margin deterioration,” Wien says. “As a result I fear that earnings will be below estimates. I have been worried about economic growth and earnings all year and the Standard & Poor’s 500 has kept working its way higher. The liquidity being provided by the Fed is responsible for that. Valuations are still reasonable and if the easy money policy continues, I recognize that stocks can move up more from here. I think earnings will be below estimates, but investors may not care as long as the Fed’s bond buying program continues.” Still, he concludes, “at some point the Fed will slow its accommodation and investors will recognize the implications of slow economic growth and very modest earnings improvement. That is why I believe a degree of caution is warranted.”